The present disclosure relates to sensors, and more particularly to gas, e.g., oxygen, sensors.
Sensors, in particular gas sensors, have been utilized for many years in several industries (e.g., flues in factories, in furnaces and in other enclosures; in exhaust streams such as flues, exhaust conduits, and the like; and in other areas). For example, the automotive industry has used exhaust gas sensors in automotive vehicles to sense the composition of exhaust gases, namely, oxygen. A sensor may be used to determine the exhaust gas content for alteration and optimization of the air to fuel ratio for combustion.
One type of sensor employs an ionically conductive solid electrolyte between porous electrodes. For oxygen detection, solid electrolyte sensors are used to measure oxygen activity differences between an unknown gas sample and a known gas sample. In the application of a sensor for automotive exhaust, the unknown gas is exhaust and the known gas, i.e., reference gas, is usually atmospheric air because the oxygen content in air is relatively constant and readily accessible. This type of sensor is based on an electrochemical galvanic cell operating in a potentiometric mode to detect the relative amounts of oxygen present in an automobile engine's exhaust. When opposite surfaces of this galvanic cell are exposed to different oxygen partial pressures, an electromotive force (EMF) is developed between the electrodes according to the Nernst equation.
With the Nernst principle, chemical energy is converted into electromotive force. A gas sensor based upon this principle typically consists of an ionically conductive solid electrolyte material, a porous electrode with a porous protective overcoat exposed to exhaust gases (“sensing electrode”), and a porous electrode exposed to the partial pressure of a known gas (“reference electrode”). Sensors used for automotive applications typically employ a yttria stabilized zirconia based electrochemical galvanic cell with porous platinum electrodes, operating in potentiometric mode, to detect the relative amounts of a particular gas, such as oxygen for example, that is present in an automobile engine's exhaust. Also, a typical sensor has a ceramic heater attached to help maintain the sensor's ionic conductivity at low exhaust temperatures. When opposite surfaces of the galvanic cell are exposed to different oxygen partial pressures, an electromotive force is developed between the electrodes on the opposite surfaces of the zirconia wall, according to the Nernst equation:
      E    =                  (                              -            RT                                4            ⁢            F                          )            ⁢              ln        ⁡                  (                                    P                              O                2                            ref                                      P                              O                2                                              )                          where    ⁢          :            E    =          electromotive      ⁢                          ⁢      force            R    =          universal      ⁢                          ⁢      gas      ⁢                          ⁢      constant                  F      =              Faraday        ⁢                                  ⁢        constant              ⁢                      T    =          absolute      ⁢                          ⁢      temperature      ⁢                          ⁢      of      ⁢                          ⁢      the      ⁢                          ⁢      gas                  P              O        2            ref        =          oxygen      ⁢                          ⁢      partial      ⁢                          ⁢      pressure      ⁢                          ⁢      of      ⁢                          ⁢      the      ⁢                          ⁢      reference      ⁢                          ⁢      gas                  P              O        2              =          oxygen      ⁢                          ⁢      partial      ⁢                          ⁢      pressure      ⁢                          ⁢      of      ⁢                          ⁢      the      ⁢                          ⁢      exhaust      ⁢                          ⁢      gas      Due to the large difference in oxygen partial pressure between fuel rich and fuel lean exhaust conditions, the electromotive force (EMF) changes sharply at the stoichiometric point, giving rise to the characteristic switching behavior of these sensors. Consequently, these potentiometric oxygen sensors indicate qualitatively whether the engine is operating in fuel rich or fuel lean conditions, without quantifying the actual air to fuel ratio of the exhaust mixture.
In addition to oxygen, the exhaust gas contains many components including carbon monoxide, carbon dioxide, hydrogen, water, nitrogen oxides, nitrogen, and a variety of hydrocarbons and hydrocarbon derivatives. Because the exhaust gas is a non-equilibrium mixture containing products of incomplete combustion, the oxygen partial pressure is not an equilibrium pressure. Because the oxygen partial pressure is not at equilibrium, sensors do not operate at stoichiometric air to fuel ratios per the Nernst equation. In addition, the use of zirconia-based electrolyte materials contributes to non-ideal sensor behavior.
To provide a means of monitoring the cell potential and circumvent at least some of the difficulties associated with non-equilibrium conditions, catalytic electrodes are used to both catalyze the oxidation reactions and to equilibrate the local oxygen concentrations. Ideal sensors produce a sharp EMF or voltage step at a stoichiometric air to fuel ratio per the Nernst equation. Manufactured sensors, however, exhibit non-ideal behaviors, for example, a broadened voltage transition that occurs over a range of air to fuel ratios near the stoichiometric ratio. In addition, the sensor EMF may depend upon mass transport processes, adsorption, desorption and chemical reactions that occur at the electrodes. There is some evidence that broadened voltage transitions and non-ideal behavior are due to a loss in catalytic activity of the electrodes. Below about 600° C., the sensor internal electrochemical factors such as electrode polarization and electrode impedance also contribute to non-ideal behavior. Many commercial sensors cease to function at about 400° C.
In order to improve sensor performance characteristics, electrolytic and chemical conditioning techniques have been utilized. Electrolytic conditioning typically involves applying static or pulsed currents to a heated sensor element in a reducing or air environment. Chemical conditioning typically consists of acid treatment (e.g., with HF, HCl, HNO3, etc.) and noble metals or noble metal salts (e.g., Pt, Rh, H2PtCl6, etc.) followed by heating to accomplish the reactions. Many prior conditioning treatments require up to six treatment steps. While suitable for their intended purpose, such treatments are costly to perform, often require the use of toxic chemicals such as hydrogen fluoride, and are inherently limited by the quality of the electrolyte and electrode films.
There thus remains a need for additional sensors and methods for producing and treating sensors elements that improve the response characteristics of the sensor.